At present times, the production from renewable energy sources being steadily growing, “conventional” power plants will increasingly be required to take on additional tasks such as to provide complementary electricity production to the grid they are connected to on short notice, particularly in the absence of large-scale energy storage systems, which are still far away from commercialization. Large fluctuations during the day require power generators to react quickly to maintain the balance between demand and production. Under these circumstances, the power plants have to supply power to the grid in a flexible way: for example, when the energy required by the grid is low they must be able to reduce the power supplied to the grid up to zero and when the grid requires power again they must be able to provide it very quickly (in some cases they must be able to provide tens of megawatt in seconds).
In the last ten years, the key area of focus of conventional power sources has been the switch from base load to intermediate load operation, and thus the need for fast load ramps, shorter low-load and start-up times, and grid stabilization. In addition, the demand for ancillary services such as provision of control reserves and frequency support, as well as tertiary control reserves and load-follow operation, has increased significantly. As a result, new operating requirements have emerged, such as two-shift operation, load-follow operation, island operation, black start capability, frequency support and very high start-up and operating reliability, in order to stabilize power grid dynamics and hence ensure secure and economic electricity supply.
As the requirements for load cycling are changing and the expansion of renewables is increasing, “conventional” power plants will have to accommodate to periods in which there is an over- or under-capacity of power. Depending on the country and power grid concerned, various dynamic capabilities are required to ensure security of supply, such as primary control, secondary control, capability for island operation, load rejection, black start capability, grid restoration following blackout, frequency stabilization, etc. Combined-cycle plants (i.e. power plants comprising gas and steam turbines) allow faster load changes within a wider load range, which make these plants more flexible. Furthermore, when considering fast start-ups and efficiency, the combined cycle power plant stands high in comparison with other electricity production methods. Even more, combined-cycle plants offer a significantly higher rate of load change than other conventional power plants thanks to innovative and specifically developed systems.
If, in future, the renewable capacity that is currently planned becomes operational, previously base loaded power plants, such as combined-cycle power plants, will not merely have to be run down to part load, but will have to be completely shut down in many cases in order to avoid significant overcapacities. These combined-cycle power plants will then need to be started up from the shut-down condition as rapidly as possible to cover demand in the event of short term loss of renewable power. The only solution, in the absence of adequate storage systems, is the increased use of conventional plants in so-called “two-shift operation”, that is, start-up and shut-down on a daily basis (and sometimes several times per day) in order to compensate for fluctuations in load. Under these operating conditions, it is essential that start-ups are able to take place very rapidly and reliably, which is possible with combined cycle plants, due to the relative simplicity of their fuel and combustion systems.
As it was mentioned, start-up reliability is becoming an increasingly important issue and combined-cycle plants exhibit significant advantages over other conventional technologies in this respect, due to the fact that they have the lowest degree of complexity. Several start-up methods for combined-cycle power plants are known in the state of the art, as per EP 2423462 A2, EP 0605156 A2, CN 202230373 U, for example. Enhanced start-ups are known as per US 2005/0268594 A1, US 2009/0126338 A1 or WO 2012/131575 A1, for instance.
It is also known in the state of the art, as per EP 2 056 421, a method to connect a combined power plant (with gas turbine and steam turbine) to a grid.
As already stated, with the deregulation of the electricity market, high fuel prices and emerging renewable energy, more combined-cycle power plants are operated as peak load power plants, which can therefore adjust the power they supply as quickly as it is needed.
Thus, the power plant manufacturer must design the combined-cycle power plants not only for base-load operation, but also for medium-load or minimum possible load operation, and especially having the increased flexibility that is required for providing the required performance as quickly as possible situations such as the shutdown of the power plant when low energy requirements are required from the power plant or for a quickly start-up for the plant providing the required performance needed.
As any start-up from standstill of the power plant is linked to a certain risk of delay or decreased reliability, the power plant operators prefer not to start the power plant from a complete shutdown operation but from a minimum load operation status. Such an operational strategy would provide the opportunity to be capable of providing a correct schedule of the power plant to meet the requirement of the electrical network, particularly providing reliable start-up within a given time window. However, operating the power plant at the minimum possible load has several problems:                The de-loading of the power plant to the minimum load operation leads to the over-stressing of the steam turbine materials. Below certain gas turbine operation range, reduced gas turbine exhaust temperature leads to reduction of the Heat Recovery Steam Generator (HRSG) steam temperature, consequently leading to cooling hot steam turbine materials such as the rotor and the admission valves.        The gas turbine is not capable to provide grid frequency support, for example, primary response. Operation at minimum load does not fulfill the transmission system requirements.        
Therefore, there exists the need of the transmission system operator to be capable to use the provided minimum plant power. The plant must be also capable to reduce the load as low as possible. The present invention is oriented towards providing the aforementioned needs.